The known underreamer has a hollow body, with a longitudinal axis, which has an external wall and which is arranged so that a pressurised drilling fluid can pass through it. At least two cylindrical bores are provided through the internal wall and their axes are transverse to the aforementioned longitudinal axis, the bores being distributed on the periphery of the hollow body. In each of the bores, there is mounted a cylindrical enlarging arm which has a cutting end, disposed on the external side of the hollow body and provided with blades for enlarging the hole. The active cutting elements deployed on the cutting end of the arms may be comprised of Polycrystalline Diamond Compacts (PDCs), rolling cutters, natural diamond cutting elements, carbide inserts, or impregnated diamond elements, or any combination thereof. Essentially any hard and abrasion resistant material known in the art can be deployed on the arms to accomplish the cutting activity as dictated by the composition of the rock to be drilled.
Each arm is arranged in the bore like a hydraulic piston, so as to be able to slide therein in the direction of its axis between an active position in which this cutting end is distant from the hollow body, in order to effect an enlarging of the hole, and an inactive position at least close to, or flush with, the external periphery of the hollow body or retracted therein. The other end of the arm, inside the hollow body, is intended to receive from a drilling fluid, circulating in the hollow body, a pressure capable of pushing the said arm into its active position. An underreamer of this type is known through the document BE-1 012 545.
In this known underreamer, before it is brought into service on each occasion, each arm may be locked distinctly in the inactive position. This is because the arms cannot be deployed unintentionally following variations in the pressure of the fluid passing through the underreamer, except as from a time chosen by the operator. In particular therefore, for each new use of the known underreamer, a new locking pin may be easily installed at the field location, in order to renew the distinct locking means.
Thus there is amongst other things a need to be able to easily reestablish this locking of the arms in a sure fashion each time the underreamer is sent into the bore hole.
In addition, this type of equipment is subjected to very harsh forces under working conditions which are known to be very difficult and therefore very expensive. Firstly, an equipment breakdown may cost enormous sums in time lost in attempting to save the equipment, for example jammed at a great depth, and in particular saving the bore hole made at great expense and which, otherwise, could be definitively condemned. Secondly, when the equipment is recovered, an equipment breakdown must be able to be repaired very easily because the technical repair means available on or close to a drilling platform are limited.
Accordingly, it is desired to procure an underreamer of simple design and reliable operation, whose arms can be locked easily and rapidly in the inactive position, and therefore without significant dismantling, and which is composed of a reduced number of parts assembled robustly, easily and rapidly dismantled and exchanged when needed.